defined as the product of orthonormal 4c AO basis functions
\(X_{\mu }(\mathbf {r}) X^{\text{RKB}}_{\mu }(\mathbf {r})\) 
fulfilling a restricted kinetic balance (RKB) condition in their
small-component, [1]} and 4c operators associated with the electronic
charge density (\(\Sigma _0\) ) and spin densities (\(\Sigma _{1-3}\) ).
The latter ones are defined via the \(2\times 2\)  zero matrix (\(\mathrm {\mathbf {0}}_{2}\) ), identity matrix
(\(\sigma _{0}\) ), and Pauli spin matrices
(\(\sigma _1,\sigma _2,\sigma _3\) ).
Note that prior to their use in Eq. (REF ), both Fock and density
matrix are transformed into the basis of ground-state molecular orbitals,
obtained from the solution of self-consistent field equations.
